Date Published: August 2, 2017
Publisher: Public Library of Science
Author(s): Marc Simó-Riudalbas, Margarita Metallinou, Philip de Pous, Johannes Els, Sithum Jayasinghe, Erika Péntek-Zakar, Thomas Wilms, Saleh Al-Saadi, Salvador Carranza, Ulrich Joger.
The Hajar Mountains of south-eastern Arabia form an isolated massif surrounded by the sea to the east and by a large desert to the west. As a result of their old geological origin, geographical isolation, complex topography and local climate, these mountains provide an important refuge for endemic and relict species of plants and animals. With 19 species restricted to the Hajar Mountains, reptiles are the vertebrate group with the highest level of endemicity, becoming an excellent model for understanding the patterns and processes that generate and shape diversity in this arid mountain range. The geckos of the Ptyodactylus hasselquistii species complex are the largest geckos in Arabia and are found widely distributed across the Arabian Mountains, constituting a very important component of the reptile mountain fauna. Preliminary analyses suggested that their diversity in the Hajar Mountains may be higher than expected and that their systematics should be revised. In order to tackle these questions, we inferred a nearly complete calibrated phylogeny of the genus Ptyodactylus to identify the origin of the Hajar Mountains lineages using information from two mitochondrial and four nuclear genes. Genetic variability within the Hajar Mountains was further investigated using 68 specimens of Ptyodactylus from 46 localities distributed across the entire mountain range and sequenced for the same genes as above. The molecular phylogenies and morphological analyses as well as niche comparisons indicate the presence of two very old sister cryptic species living in allopatry: one restricted to the extreme northern Hajar Mountains and described as a new species herein; the other distributed across the rest of the Hajar Mountains that can be confidently assigned to the species P. orlovi. Similar to recent findings in the geckos of the genus Asaccus, the results of the present study uncover more hidden diversity in the northern Hajar Mountains and stress once again the importance of this unique mountain range as a hot spot of biodiversity and a priority focal point for reptile conservation in Arabia.
Understanding and quantifying biological diversity is imperative if we want to be able to explain and, ultimately, conserve it. Nevertheless, despite more than 250 years of taxonomic work, we still lack precise estimates of the number of species living on the planet. This uncertainty is typically attributed to inadequate knowledge of small-sized species with reduced distribution ranges concentrated in hotspots and less explored areas of our planet such as the deep sea and soil [1,2]. Recently, molecular methods have revealed that cryptic species, that is, two or more morphologically similar species erroneously classified under the same species’ name, contribute another layer of uncertainty toward realistic estimates of our planet’s diversity [3,4]. One important aspect of crypsis is its potential impact on conservation planning: species that are considered common, widely distributed and of low conservation priority may actually contain multiple morphologically similar species, each with small ranges and therefore of high conservation concern [5–7]. Given the great need for cataloguing and conserving our biota, uncovering cryptic species should be a high priority research so that many of these evolutionary units are not lost before they can be described .
The results of the integration of molecular, morphological and ecological data reveal a very old speciation event within the genus Ptyodactylus and highlight another case of endemicity in the northern tip of the Hajar Mountain range . Our discovery is relevant because it shifts attention from the much better explored and studied Jebel Akhdar, in the central section of the Hajar Mountains, a recognized hotspot of biodiversity [78,79] and indicates that other less investigated areas of the Hajar Mountains may be reservoirs of high levels of diversity, especially of cryptic species. With the description of Ptyodactylus ruusaljibalicussp. nov. and the presence of Asaccus margaritae, A. gardneri and A. caudivolvulus , this short and narrow mountain stretch of approximately 140 km from north to south and 40 km from east to west (4,350 km2) contains four endemic species of reptiles, exactly the same number of endemics inhabiting the Jebel Akhdar in the central Hajar Mountains (Hemidactylus luqueorum, Asaccus platyrhynchus, A. montanus and Pristurus gallagheri; ). The recent descriptions of Asaccus species  and the present work are very good examples of the potential impact that the lack of taxonomic knowledge (the Linnean shortfall; ) can have on conservation planning. Species that are considered common and widely distributed may actually contain multiple species, each with small ranges and of potentially high conservation concern, such as A. caudivolvulus, the reptile with the smallest distribution range of all 19 reptile species endemic to the Hajar Mountains [5–7].